1. Field of the Invention
The present invention relates to a conveying device for feeding printed products to a processing unit. In this device, the printed products are conveyed along a conveying path by means of a first conveyor, from a stack in which the printed products are arranged vertically, to a second conveyor which has a higher circumferential speed than the first conveyor for forming an imbricated formation which is upwardly offset, wherein a holding-down device which acts on the printed products and is formed of at least one endless traction means is arranged in a transition area formed by the conveying end of the first conveyor and the conveying beginning of the second conveyor.
2. Description of the Related Art
Devices of the above-described type are used for feeding printed products in businesses which further process printed products to gathering machines, gather-stitchers and insertion machines. Printed products are products such as printed sheets, cards, CD/DVD-ROM, large-surface product samples, etc., whose dimensions may vary within a wide range.
For this purpose, the printed products to be processed are loaded onto the conveying device. Depending on the type of delivery of the printed products, loading takes place, for example, manually or by means of lifting tools. Subsequently, the printed products are placed in the form of a horizontal stack with one of the side edges onto an essentially horizontally extending first conveyor which feeds the printed products as necessary to a subsequently connected second conveyor. This second conveyor has the purpose of converting the stack-shaped formation of the printed products on the first conveyor into an imbricated formation, and to feed the printed product subsequently in this preferred formation to a magazine of a processing unit. For this purpose, the first conveyor forms with the second conveyor an obtuse angle and the speed of the second conveyor is substantially higher than that of the first conveyor. The imbricating function is based on the friction principle. The aim is to form an imbricated formation which is as uniform as possible in order to achieve the best conditions for an optimum operation of the feeder of the processing unit. The uniformity should be such that printed sheets which are resting against each other are at least shifted relative to each other and no gaps are created in the imbricated formation. In addition, the printed products may not be pulled obliquely in relation to the conveying device while the imbricated formation is being formed. The formation of the imbrication from the horizontal stack into a conveying device is the central function. To achieve this, additional organs and functions are necessary for supporting the effect of the two conveyors. In the end area of the first conveyor it is possible to provide on both sides of the stack further conveying units in the form of rollers, tapes or chains which slightly upset or pre-separate the printed products transversely of the conveying direction. In addition, in conveying devices according to the state of the art, a holding-down device is provided composed of several circumferentially traveling traction means which extend beyond the end area of the first conveyor or up to the inclined front section of the second conveyor, wherein the printed products are grasped at their upper side edges and pressed against the lower conveying element of the first conveyor and the front section of the second conveyor. The conveying plane formed by the first conveyor is fixedly defined, so that inevitably the holding-down device must be adapted to the height of the printed products and also to the thickness of the produced imbricated formation. In a holding device according to the prior art, this results in an adjustment of the position of the holding-down device in the direction of two axes. However, this adjustment is time-consuming and critical because it significantly influences the imbricated formation. In addition, due to changes of the speed conditions between the first and second conveyor, the conveyors must be adapted to the changing circumstances, for example, while the thickness of the imbricated stream changes due to changes of the speed conditions between the first and second conveyors.